(1) Field of the Invention
The present invention relates to a novel structural element and more particularly to a structural component comprising a pair of rim members connected to each other by means of a plurality of spokes arranged such that upon application of a force acting to separate the rims, tension loads are produced in the spokes and compression loading is produced in the rim members thereby providing a balanced structure having substantial strength for its weight.
(2) Description of the Prior Art
One of the primary goals of structural engineering is to produce an "efficient" structure, i.e., a structure which uses the least possible amount of material commensurate with meeting preselected design parameters. Normally this is accomplished by refining the individual structural elements such that they carry only simple tension or compression loads, e.g., replacing a rectangular beam with a truss or replacing a beam with an arch. More recent examples of developments in the structural element area are geodesic domes and large fabric structures. The present invention represents a new type of structural element having many possible applications for resolving a broad range of existing structural problems of which the following are merely exemplary.
Heretofore, wheels have been made using either a heavy single piece dish attached to a rim, as seen in a typical automobile wheel, or using a hub, spokes and a relatively narrow rim such as in a bicycle wheel. Wide wheels had to be of relatively solid, heavy construction. Such a heavy, one-piece rim leaves the resulting wheel with high mass and rotational moment of inertia which decreases acceleration and fuel efficiency. Large aircraft, in particular, would benefit from large, low mass wheels.
Artificial ocean islands, such as those used as oil production platforms in the Arctic Ocean, in order to be built without a caisson-type structure to reinforce and contain the fill, must have gentle side slopes on the order of 15 to 1. The amount of fill material required to construct such an island however increases exponentially as a function of water depth. For water over 30 meters deep, such uncontained islands require so much fill that they are no longer considered economical. Previously, caisson contained and reinforced islands have been built, each requiring relatively massive steel or concrete structures for containing the fill and helping to distribute loads from ocean wave, ice and ship impacts against the island's foundation. The primary disadvantages of such rigid caisson-type structures are; a large quantity of steel and concrete is required to form the structure, the large noncollapsible shape is difficult to construct and to transport to the site, and the structure is less efficient in that it requires additional material to provide the required strength. To reduce the amount of material required, such caissons are normally placed on top of an artificial sea mount. This however, exposes the island to a failure mode wherein the caisson may be shoved off the mount. In some cases, a sheet pile wall or group of sheet pile cells is used to contain the fill. Sheet pile caissons, however, require a long period of good weather on site to permit driving of the piles.
Bucket or basket-type containers use tubular or tension fabric construction to contain the fill material and to distribute the lift forces. For extremely large containers, such as a dead weight anchor, tubes with their solid side walls and flat bottoms are not structurally efficient. Significant amounts of material are required to resist bending stress in both the flat bottom disc, and the connection between the bottom and the side walls. Tension fabric containers are more structurally efficient. However, their smooth rounded shape offers little resistance to dragging across the ocean bottom when a large lateral force is applied to the anchored structure.
Present above ground fluid tanks are essentially tubes. For oil storage such tanks are generally short fat tubes. One factor which limits their economical size is the wall thickness that is required to prevent buckling failure when large lateral forces, such as those generated by an earthquake, are applied. The traditional all-welded, steel oil tank must be built on site. This obviates the savings and faster deployment possible with a prefabricated collapsible structure.
Towers and tall buildings transfer lateral forces, e.g., from wind and earthquake, to their foundations by guy wires, diagonal truss bracing, shear walls, or moment resisting frames. The primary disadvantage of guy wires is the amount of space they require, usually being an area swept out by a radius as long as the tower is high. Diagonal bracing requires more materials, more connections and may in some cases make a structure too rigid thus forcing it into the same resonant frequency as the lateral force. Such bracing only works well on flat-sided structures with three or four sides. Solid shear walls are too massive for some purposes, although they previously were considered the most reliable structural component for ensuring earthquake survival in buildings having less than twenty stories. Moment resisting frames achieve their structural efficiency based on the principal that they are expected to partially fail without actually collapsing during large earthquakes. This planned failure acts to absorb energy so that lives are not lost; however, the building then has to be replaced.
To date, fixed ocean platforms have used either a rigid structure such as a steel jacket or concrete tube, or used compliant strength means such as seen in tension leg platforms (TLP) or guyed towers. Such rigid structures require larger amounts of material and become prohibitively expensive as water depth and environmental forces increase. Compliant oil production structures such as TLP's also have the disadvantage that their lateral motions must be no greater than 5% of the water depth to avoid breaking the drill strings used therewith which then forces them into a resonant frequency near to that of ocean waves in shallower water thus greatly magnifying the forces.
What is thus required is a light weight, high efficiency structural element adaptable to a wide variety of constructions and environments.